Late at night, the speaker can't stop thinking about someone who is no longer part of their life, or at least not close by. The poem doesn't say what happened between them, and it doesn't need to. What matters is the effect: shared memories keep coming back, and the speaker is too tired and too raw to fight them.
The most striking image is the memories "pounding" the speaker's head. Memories are usually described as soft or sentimental, but here they feel like blows, something that hurts and won't let up. The shift in the last lines is just as telling. The speaker stops talking about the person and starts snapping at their own mind: "Just shut up, / And let me sleep." It sounds like someone worn out, almost pleading.
The mood is restless, weary, and quietly painful. The short lines and plain wording make it feel like a thought caught at 2 a.m., unpolished and honest. The title's "(2)" hints this may be one of several midnight poems, though the text itself doesn't say more.